Abstract: The biological denitrification of water is improved in a decisive manner by reducing on the one hand the oxygen content of the water prior to passing it through the denitrification state and, on the other hand, by bringing the water into contact with the denitrification bacteria in a fluidized bed formed by sand the grains of which have been populated intentionally with denitrification bacteria of the species "pseudomonas fluorescens". Accordingly, the apparatus for carrying out the method comprises a stage (51) for eliminating oxygen preceding the denitrification reactor (52). If this stage is designed as a degassing stage, the reactor (52) is followed, advantageously, by a decarbonization stage (81).